Kinetics of the Photochromic Decay Reaction of Solutions of 2,2′,4,4′,5,5′-Hexaphenyl bi-imidazolyl

Abstract

THE photochromism, in solution and in the solid state, of the compound formed by the oxidation of 2,4,5-tri-phenylimidazole (lophine) using potassium ferricyanide was discovered by Hayashi et al.1. Thermochromism was also observed in solution and the reddish-purple colour of the solution so formed was attributed to the formation of the radical, 2,4,5-triphenylimidazolyl (L.). After allowing for thermochromism, Hayashi found that the photochromic decay reaction obeyed first order kinetics with an activation energy of 14 kcal/mole, and Ueda2 studied the hyperfine splittings of the radical, and confirmed the first order kinetics. Further work3 by Hayashi indicated that the kinetics of the reaction were second order, suggesting that the reaction was a simple bimolecular radical recombination. This communication reports a more detailed examination of the kinetics which shows that the kinetic order is 3/2, falling to first order, which suggests a complex mechanism.